This invention relates to in situ recovery of shale oil, and more particularly, to techniques involving the excavation and explosive expansion of oil shale formation in preparation for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598 and 4,118,071 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary, fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
Techniques used for explosively expanding formation toward the void space in a retort site can affect the permeability of the fragmented mass. It is desirable to form a fragmented mass having a distribution of void fraction suitable for in situ oil shale retorting; that is a fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass through which oxygen-supplying gas can flow relatively uniformly during retorting operations. Gas channeling through the fragmented mass can occur when there is non-uniform permeability.
A fragmented mass of reasonably uniform void fraction distribution can provide a reasonably uniform pressure drop through the entire fragmented mass. When forming a fragmented mass, it is important that formation within the retort site be fragmented and displaced, rather than simply fractured, in order to create a fragmented mass of generally high permeability; otherwise, too much pressure differential is required to pass a retorting gas through the retort. Preferably the retort contains a reasonably uniformly fragmented mass of particles so uniform conversion of kerogen to liquid and gaseous products occurs during retorting. A wide distribution of particle size can adversely affect the efficiency of retorting because small particles can be completely retorted long before the core of large particles is completely retorted.
The general art of blasting rock formations is discussed in The Blaster's Handbook, 15th Edition, published by E.I. DuPont de Nemours & Company, Wilmington, Del.
One method of explosive expansion involves use of a plurality of concentrated charges uniformly distributed throughout the formation to be expanded to produce a generally uniformly fragmented mass of formation particles. U.S. Pat. No. 3,434,757 to Prats teaches sequential detonation of a series of explosives in oil shale to form a permeable zone in the oil shale.
The aforementioned U.S. Pat. No. 4,118,071 discloses techniques for fragmenting a volume of formation containing oil shale to form a fragmented permeable mass in an in situ oil shale retort. In that patent, an in situ oil shale retort is formed in a subterranean formation containing oil shale by excavating a void in the form of a narrow slot having a vertically extending free face, drilling blasting holes adjacent to the slot and parallel to the vertical free face, loading explosive into the blasting holes, and detonating the explosive to expand the formation adjacent the slot toward the free face.
In forming a fragmented mass, formation within the retort site can be explosively expanded toward a vertical slot in a single round of explosions. Since each blasting hole in the retort site can contain as much as about eight tons of explosive, significant seismic effects can be produced from the single round of explosions. It is desirable to use blasting techniques that minimize the seismic effects from the explosive.